The importance, in fitting a patient with proper corrective spectacles, of accurate measurement of the interpupillary distance, has long been recognized. For example, Brown states: "The distance between the two eyes is a matter that should not escape the attention of the careful observer, because of the disturbing effect it may have on the act of convergence.
It seems like a very simple matter to measure the distance between the eyes, and yet when we inquire into the question carefully, we will find that to make an exact measurement is attended with no little difficulty.
In the first place, the pupil is not exactly in the center of the anterior part of the eye-ball,
The second element of error lies in the difficulty of measuring the exact distance between the pupils, as this is a point that can only be determined approximately." (C. H. Brown, M.D., "The Optician's Manual", Publ. by The Keystone, Philadelphia, Copr. 1897, pp. 187-188).
The significance of the interpupillary distance is that it determines the distance between the optical axes of the two eyeballs. In any corrective lens, it is important that the optic axis of the lens (or portion thereof, in the case of bifocal or multifocal lenses) coincide with the optic axis of the eyeball before which it is placed. Otherwise, a prismatic error is introduced, in that, in order to fix upon an object, the eye is constrained to fix slightly off-center, rather than directly through the optical center of the lens. As a result, a prismatic error is introduced as a ray of light from the object passes angularly (instead of normal) to the surface of the optically denser medium of the lens, and a second error, of opposite sign, when the ray emerges from the dense medium of the lens to the less dense medium of the atmosphere between the lens and the eyeball. When the lens approaches a biplanar surface, as in lenses of less than 1 diopter strength, the two errors tend to cancel each other, and no serious problem is introduced. When stronger lenses are involved, however, the opposed lens surfaces depart more and more from the configuration of two parallel planes, and the residual, uncancelled prismatic errors may become quite serious. For example the New Jersey State Board of Opticians, for purposes of its qualifying examination, accepts a prismatic error of no greater than plus or minus 0.25 prism diopters in each lens. Considering that existing methods of measuring interpupillary distance commonly introduce errors of plus or minus 4 mm. in interpupillary distance, taken together with the fact that an error as small as 1 mm., in measurement of interpupillary distance for a plus 5 diopter corrective lens, introduces a prismatic error of 0.5 prism diopters (sometimes indicated by the notation 0.5.degree. prism), it can readily be seen that large and unacceptable errors are readily introduced by faulty measurement of interpupillary distance.
The quantity ".sup.o prism" or "prism diopters" is the displacement in millimeters of an image on a screen 1 meter away from the lens, cast through the lens by a point source of light on the optical axis of the lens located 1 meter from the lens on the side opposite the screen.
Thus, in addition to the strain on the recti muscles used for convergence, when the optical axis of the lens fails to coincide with that of the eye, there is an additional disadvantage, resulting from distortion introduced by prismatic error. In extreme cases, prismatic error may result in unsharpness and color fringes, based on the prismatic dispersion of light of different wavelengths, in the same manner as in a Newtonian prism.